Juvenile howling monkeys are monomorphic during most of their juvenile period as no sexual differences are apparent until they become sexually mature. We hoped to determine if there were behavioral differences which would allow us to noninvasively determine sex. Behavior and proximity of juvenile howling monkeys from 5 different social groups on Hacienda La Pacifica were compared for age and sex differences. Focal animal observations (695.9 hours) revealed that there were no significant differences in activity pattern by age or by sex, but that there were differences in proximity to mother. Proximity to adult females were different by age, with youngest animals spending the least amount of time near adult females. There were no differences in proximity to adult males by age, but juvenile males spent significantly more time than females near adult males. There were no differences by age or sex in proximity to other juveniles, or any differences in play. Howling monkey juveniles appear to be almost as monomorphic in behavior patterns as they are